package org.bouncycastle.crypto.generators;

import java.math.BigInteger;

import org.bouncycastle.crypto.AsymmetricCipherKeyPair;
import org.bouncycastle.crypto.AsymmetricCipherKeyPairGenerator;
import org.bouncycastle.crypto.KeyGenerationParameters;
import org.bouncycastle.crypto.params.DHKeyGenerationParameters;
import org.bouncycastle.crypto.params.DHParameters;
import org.bouncycastle.crypto.params.DHPrivateKeyParameters;
import org.bouncycastle.crypto.params.DHPublicKeyParameters;

/**
 * a Diffie-Helman key pair generator.
 * 
 * This generates keys consistent for use in the MTI/A0 key agreement protocol as described in "Handbook of Applied Cryptography", Pages 516-519.
 */
public class DHKeyPairGenerator implements AsymmetricCipherKeyPairGenerator {
    private DHKeyGenerationParameters param;

    public void init(KeyGenerationParameters param) {
        this.param = (DHKeyGenerationParameters) param;
    }

    public AsymmetricCipherKeyPair generateKeyPair() {
        BigInteger p, g, x, y;
        int qLength = param.getStrength() - 1;
        DHParameters dhParams = param.getParameters();

        p = dhParams.getP();
        g = dhParams.getG();

        //
        // calculate the private key
        //
        x = new BigInteger(qLength, param.getRandom());

        //
        // calculate the public key.
        //
        y = g.modPow(x, p);

        return new AsymmetricCipherKeyPair(new DHPublicKeyParameters(y, dhParams), new DHPrivateKeyParameters(x, dhParams));
    }
}
